let arr = ['a', 'b', 'c', 'd', 'e'];

Array.prototype.my_map = function(cb) {
    const newArr = [];
    for(let i = 0; i < this.length;i++){
        const res = cb(this[i], i , this)
        newArr.push(res);
    }

    return newArr;
}





arr.map((item, index, arr) => {
   return item + '1'
})